Thank you guys,
Installed MPLAB 8.92 and I'm now able to build the default bootloader file for the 18F452 - that's progress. Next I'll look at adding a device of my own.
I wonder if there are any plans to switch the UMC stuff to MPASMX as no new devices (like the 26K40) are available for the old assembler (I think I know the answer to that....)

I guess it should be possible to "manually" build the bootloader files using MPLABX instead of the UMC builder application. But, one step at a time.

/Henrik.